Solder balls are mainly used to connect a number of electric/electronic components together. Specifically, solder balls are used as input/output terminals for a quad flatpack package (QFP) with lead terminals around its components and semiconductor packages such as a ball grid array (BGA) and a chip size package (CSP), which are relatively small in size and which can cope with multiple-pin applications.
FIGS. 10(a) and 10(b) are respectively a perspective view and a cross-sectional view of a BGA that uses solder balls. As shown in FIGS. 10(a) and 10(b), a BGA is an LSI package in which silver-coated balls 50 are bonded onto the lower surface of an LSI chip with an interposer 62 interposed between them. The silver-coated balls 50 are arranged in matrix on one side of the interposer 62, and are used as input/output terminals for the package. Each of these silver-coated balls 50 is a tiny metallic sphere with a diameter of about 0.1 mm to about 1.0 mm, and may be obtained by forming a solder layer, including lead (Pb), on the surface of the metallic ball, for example.
In recent years, solder including lead is being replaced with solder including no lead (which is also called “Pb-free solder”) in order to handle environmental issues. In view of these circumstances, the applicant of the present application disclosed a solder ball, of which the surface is coated with a tin-silver (Sn-AG) based solder layer with no lead and in which the creation of voids is minimized even when being heated and melted (see Patent Documents Nos. 1 and 2).
In general, solders are roughly classifiable, according to their soldering temperature, into medium-low temperature solders with melting temperatures of approximately 150° C. to approximately 250° C. and high-temperature solders with melting temperatures of approximately 250° C. to approximately 300° C. The medium-low temperature solder is mostly used to connect an electronic component onto a printed circuit board, while the high-temperature solder is often used to connect the interconnects of an electronic component together.
The Sn—Ag based solder layer mentioned above has a melting point of approximately 216° C., and therefore, solder balls with this solder layer can be used effectively in a soldering process at a medium to low temperature. In a high temperature range of about 250° C. to about 300° C., however, the Sn—Ag based solder layer would melt again to deform the balls, for example. For that reason, the Sn—Ag based solder layer cannot be used for a high-temperature soldering process. That is why a lead-free solder ball applicable to such a high-temperature soldering process is now in high demand.
Meanwhile, it is known that a metal in the form of nanoparticles (i.e., superfine particles with particle sizes of several nm to several hundreds of nm) has physical properties that are quite different from those of the same metal in bulk. For example, it is known that silver nanoparticles get sintered at a far lower temperature than silver in bulk. As for silver nanoparticles, Patent Document No. 3 discloses a method of making a silver colloid organosol including silver nanoparticles with a mean particle size of approximately 32 nm (see Examples of Patent Document No. 3).                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2004-114123        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2004-128262        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2003-159525        